The invention is in the field of information transmission from a well drilled in the ground up to the surface. More particularly, the invention relates to a optimized method for the transmission of informations between the bottom of a drilled well and the surface, the well either having already been drilled and is in production, or is in the process of being drilled.
Various systems are known for the transmission of informations between the bottom of a well and the surface, e.g. by mud pulses in a fluid circulating in the well. However, it is known that this type of transmission suffers from the disadvantage of not operating correctly or not operating at all in a compressible fluid, such as gas or liquids charged with gas, or when there is an obstruction in the circulation channel disturbing the flow, e.g. an underground motor, valve or nozzle. Moreover, this system is obviously inoperative during production and manipulation of the drill string.
The system of transmission by electromagnetic waves guided by metal tubings placed in the well is also known. This transmission system is more particularly described in FR 2681461 of the present applicant, cited here for reference purposes. The performance characteristics of the electromagnetic transmission (EM) are dependent on the mean resistivity of the geological formations surrounding the well. If the resistivity of certain layers is inadequate, as is the case with certain sedimentary, tertiary, peri-continental rocks like those of the North Sea or the Gulf of Mexico, the attenuation can become excessive along the well, which makes it impossible to use such a device in most offshore wells unless it is possible to accept a drastic reduction in the transmitted information flow.
Thus, the present invention relates to a method for the transmission of informations from a well drilled through geological formation layers, at least partly cased by metal tubes, the method comprising the placing in said well of an information transceiver operating by guided electromagnetic waves created by the injection of an electric signal by a dipole conductively connected to the metal tubes used for guiding the transmitted waves. According to the method, identification takes place of the attenuation of the transmission by certain formation layers having a low resistivity and there is an at least partial electrical insulation of the metal tubes positioned opposite said low resistivity layers.
With the aid of a mathematical model it is possible to determine the minimum length to be insulated, bearing in mind the minimum characteristics of said electromagnetic transmission, more particularly the transmission distance and/or the information flow rate.
Insulation can be obtained by the installation of tubes previously coated with an insulating material layer.
In a variant, insulation can be obtained by the putting into place of an insulating material of the cement type opposite said formations in the annular space between the tubes and the formations.
The transceiver can be positioned close to the lower end of a production tubing in order to transmit sea bed or bottom measurements or instructions to sea bed or bottom equipment.
Said transceiver can also be positioned close to the lower end of a drill string in order to transmit bed or drilling parameters, or tracking or location measurements.
The invention also relates to a system for the transmission of informations from a well drilled in geological formation layers, at least partly cased by metal tubes, the system comprising in said well an information transceiver operating by means of guided electromagnetic waves created by the injection of an electric signal by a dipole conductively connected to the metal tubes used for guiding the transmitted waves. In the system, at least some of the metal tubes positioned opposite the low resistivity layers have means for electrical insulation with said formation.,
The insulated tubes can be coated with an insulating material layer. The insulating layer may not cover the entire length of the tube.
In the system, the insulating means can comprise an insulating material filling the annular space between the tubes and the conductive formation, the material being the result of the hardening of a liquid composition.
The transceiver can be incorporated into the end of a production tubing.
The transceiver can also be incorporated into the end of a drill string.
The system according to the invention can be applied to an ocean drilling installation with a subsea wellhead.
In this application, a kill-line can be externally electrically insulated from the sea bed to the surface.